1. Field of the Invention
This invention relates to micro-channel plates (MCP's). The invention is concerned particularly with MCP's for use in imaging x-rays and particles having equivalent wavelengths.
2. Description of the Related Art
MCP's have been utilised to perform a lens function in x-ray and the like imaging applications. X-rays, or, particles reflected at grazing incidence from the internal glass walls of the channels, or pores, of the MCP can be brought to a focus.
Square pore MCP's have been successfully applied in focusing X-rays or particles having equivalent wavelengths, for example neutrons, and have been used for example in X-ray telescopes. Other possible uses include X-ray lithography, flux concentration for X-ray scattering experiments, neutron focusing, X-ray microscopy and in diagnostic and therapeutic X-ray machines.
The use of square pore MCPs in X-ray imaging is described in, for example, the paper entitled "X-ray focusing using micro-channel plates" by P. Kaaret et al published in Applied Optics vol. 31, No. 34, pages 7339 to 7343, 1992. In an experimental arrangement described in this paper a flat (planar) MCP is utilised to focus diverging X-rays from a point source located at a finite distance from the MCP to an image. The pores of the MCP are parallel to each other and tilted relative to the surface by a bias angle and the MCP is orientated such that the pore axes are parallel to the optical axis.
As is mentioned in this paper, square pore MCP's are considered to offer an improvement over MCP's having circular pores as they lead to a significant increase in the intensity of the focused beam which, it is said, is due to the fact that the angles of incidence and reflection are the same regardless of the point of reflection in the square geometry.
Square pore MCP's for X-ray and the like imaging have also been produced in a spherically curved configuration in which the axis of each pore is aligned radially with respect to a spherical surface. By arranging that the axes of the pores extend normal to the spherical surface in this manner, parallel rays from a source at infinity can be imaged. The use of such an MCP is reported in the paper entitled "X-ray focussing using microchannel plates" by G. W. Fraser et al published in SPIE Proceeding, Vol. 1546, page 41-52, 11991.
In these MCP's the pores are square-packed, that is to say, in cross-section, the pores are arranged in othogonal rows and columns, in a grid like pattern.
We have found that improved results are achieved with a different arrangement.